We are comparing legume- and grass-based beef production systems in terms of greenhouse gas emissions from animal urine and dung, and potential methane emissions from enteric fermentation. Typically enteric fermentation emissions in legume-based diets are less than those of animals grazing pure grass pastures, however greater nitrogen content in diets containing legumes may increase emissions of some greenhouse gases (such as nitrous oxide) from animal excreta. The first year of field experimentation was completed in October 2015, and laboratory and data analysis are ongoing.
- To quantify emissions of N2O and CH4 from animal dung and N2O from animal urine for these two pasture types in order to obtain emission factors of N2O separately for each excreta type (urine or dung) and for animals with contrasting diets, and to compare emission factors obtained in this study with others reported for different production systems in the literature and those used by the IPCC model;
- To quantify enteric fermentation CH4 emissions from the two pasture types and compare these measurements with those estimated by the IPCC model with parameters currently used for Southeast US in order to assess the prediction capability of the model;
- To sum emissions of N2O from dung and urine and CH4 from dung and enteric fermentation of animals grazing the two pasture types and use global warming potential to estimate emissions in CO2e.
A 2-yr experiment was conducted at the University of Florida Beef Research Unit (BRU) in Gainesville, FL (29.72 N°, 82.35°W) during the summers of 2015 and 2016. Source pastures for animal excreta were ‘Pensacola’ bahiagrass monoculture or mixed ‘Florigraze’ rhizoma peanut-bahiagrass pastures. Proportion of rhizoma peanut was 45 to 67% in 2015 and 73 to 78% in 2016. Bahiagrass pastures were fertilized with 50 kg N ha-1 in late spring/early summer of both 2015 and 2016. Treatments consisted of two forage systems (N-fertilized grass and grass-legume) and two animal excreta types arranged as a factorial experiment in four replicates of a completely randomized design. Sixteen static chambers were used to accommodate the four replicates of four treatments, and two extra chamber were utilized as blanks to account for background emissions. The chambers were circular, with a radius of 30 cm (0.283 m2) for urine and 15 cm (0.071 m2) for dung to mimic typical area occupied by each excreta type (White-Leech et al., 2013). Bases were removed between the first and second year of the experiment and reinstalled at a different location in the same pasture to avoid any carryover effect of first-year excreta application on second-year GHG emissions.
Animal excreta was collected after animals grazed either pasture source for 1 wk. Within a source treatment, dung was thoroughly mixed and weighed. Urine was applied at a rate of 2 L and dung at 2 kg fresh weight per chamber (Haynes and Williams, 1993). Excreta collection occurred on 3 and 4 Aug. 2015 and 5 and 6 July in 2016. Excreta application occurred on 5 Aug. 2015 and 7 July 2016. Gas sampling started on the day of excreta application. Sampling frequency was daily in the first week, 3 times a week in weeks 2 and 3, and 2 times a week in weeks 4 and 5. Emission factors of N2O were calculated for each treatment (type of excreta and type of pasture source) as percentage of N emitted as N2O. Data were analyzed using a mixed model where treatments (pasture source for CH4 because only emissions from dung were quantified; and pasture source times excreta type for N2O) were considered fixed and year was considered random.
There was an interaction of sampling day (number of days after excreta deposition) and source pasture on CH4 flux (P = 0.0084). Emissions were greater from dung of animals grazing RP-BG on Days 2, 3, and 4 when compared with those grazing BGN pastures. In dung from animals grazing RP-BG pastures, emissions were greater in the first 3 d compared with the period starting at 6 d after treatment application (P < 0.0078), with intermediate emissions at Day 4 and 5 (P > 0.0525). When dung pasture source was BGN, there was no day effect on CH4 flux during the sampling period (P > 0.1089). As flux of CH4 emissions was greater from dung of animals grazing on RP-BG pastures, cumulative emissions were also greater in this treatment relative to BGN (P = 0.0034; 39 and 69 mg CH4-C [dung pile]-1). If we consider an average number of 17 defecations per day (Aland et al., 2002), we estimate annual emissions of 241 and 422 g CH4-C animal-1 yr-1 for animals grazing BGN and RP-BG, respectively. Therefore, CH4 emissions from animals grazing legume-rich pastures was 1.75 times greater than those grazing bahiagrass fertilized with a typical fertilization regime for extensive production in the southeastern U.S. This difference is likely related to the greater N concentration of dung from animals grazing RP-BG compared with BGN (2.9 and 1.8 g kg-1, respectively).
Nitrous oxide emissions
Flux of N2O-N was affected by excreta type (P = 0.0144), with greater average daily emissions from urine compared with dung (0.26 and -0.10 mg N2O-N m-2 h-1, respectively). This resulted in different cumulative emissions for the two excreta types (P = 0.001), with urine having greater cumulative emissions compared with dung (45 vs. 0.8 mg patch-1, Figure 5-6). Considering a N input of 2.1 and 4.7 g N (excreta patch)-1 for dung and urine, respectively, we found EFdung = 0.02% and EFurine = 2.14% N input emitted as N2O-N. Therefore, N2O-N emissions from urine were around 120 times greater than from dung in our study. This difference was probably related to more NH4+ available from urine relative to dung (Sordi et al., 2013). Our results support the work of others suggesting that emissions from urine and dung in grazing systems should be accounted for separately (van der Weerden et al., 2011).
Effect of pasture management on overall GHG emissions
Pasture composition did not affect cumulative emissions of N2O-N from cattle urine or dung. Therefore, considering 17 defecations and 9 urinations per day (Aland et al., 2002), cumulative N2O-N emissions on an annual basis were 5 and 148 g N2O-N animal-1 yr-1 for dung and urine, respectively. Dung CH4-C emissions from animals grazing RP-BG were greater than from animals grazing BGN (422 and 241 g CH4-C animal-1 yr-1). The GWP, i.e., the ability of a gas to trap energy when compared with CO2, is 265 for N2O and 28 for CH4 (IPCC, 2014). If we consider a stocking rate of two animals ha-1, and transforming emissions to CO2eq based on each gas GWP, GHG emissions from animal excreta are 127 and 116 kg CO2eq ha-1 yr-1 for RP-BG and BGN, respectively. Direct emissions from N fertilizer can be estimated to be 0.5 kg N2O-N yr-1 using an emission factor of 1% (de Klein et al., 2006), which is equivalent to 208 kg CO2eq ha-1yr-1. Therefore, annual emissions from BGN are 2.5 times greater compared than those from RP-BG (324 and 127 kg CO2eq ha-1 yr-1, respectively), and this difference is mostly due to emissions from N fertilizer. This difference could potentially be greater, since in these estimates we are not accounting for other emissions associated to N fertilizer (de Klein et al., 2006), i.e., emissions from fertilizer production, storage, and transportation, and from fuel necessary for its application (Lal, 2004). In addition, CH4 emissions from enteric fermentation of animals grazing tropical legumes are estimated to be 20% less than those from animals grazing C4 grasses (Archimède et al., 2011). These results indicate that the inclusion of legumes in grass swards is a viable alternative to improve sustainability of grazing systems in terms of GHG emissions.
Educational & Outreach Activities
Results of this project were presented to the Perennial Peanut Producers Field Day in 2016 and 2017 to approximately 200 producers. Results were also discussed at the annual Florida Forage Workers’ Tour with approximately 50 professionals and at the Crop Science Society of America meeting in 2016 and 2017 to about 100 professionals total. Tours were given to visiting scientists at the research unit and content was incorporated into the course Tropical Grassland Agroecosystems. Marta Moura Kohmann presented preliminary data on this project at the 2016 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America Annual Meeting in Phoenix, AZ and will present the final data set at the 2017 meeting in Tampa in October 2017. References for these abstracts are listed below. The work of this project was included in her thesis entitled “Rhizoma Peanut Proportion in Mixed-Species Pastures with Bahiagrass Affects Nutrient Cycling and Greenhouse Gas Emissions”, to be defended in October 2017. In addition, results will be published as an original research paper (in preparation).
Kohmann, M.M., L.E. Sollenberger, J.C.B. Dubeux, Jr., N. DiLorenzo, M.O. Bauer, S.S. Santana, and L.S.B. Moreno. 2016. Effect of legume-rich and N-fertilized grass diets on greenhouse gas emissions from animal excreta. ASA/CSSA/SSSA National Meeting Abstracts, Phoenix, AZ.
Kohmann, M.M., L.E. Sollenberger, J.C.B. Dubeux, Jr., N. DiLorenzo, M. Bauer, S. Saraiva, L.S.B. Moreno, L.S. da Silva, and E. Stenklyft. 2017. Effect of legume-rich and N-fertilized grass diets on nitrous oxide and methane emissions from bovine excreta. ASA/CSSA/SSSA National Meeting Abstracts, Tampa, FL.
Through this study, we learned that dung from animals grazing legume-grass pastures emitted more CH4-C than dung from animals grazing grass pastures, but there was no effect of forage treatment on N2O-N emissions from animal excreta. We also confirmed that emissions of N2O-N were greater from urine than dung, supporting a conclusion that they should be accounted for separately in model estimates. Nitrous oxide emission factors were 2.14% for urine and 0.02% for dung in our study; this factor for dung being much lower than the 2% indicated by IPCC (2006) and EPA (2016) for the combination of both excreta types. This indicates that N2O emission estimates from animal excreta of grazing livestock production systems may be overestimated by up to three times. Although CH4-C emissions were greater from dung of animals grazing legume-grass than grass pastures, when accounting for both animal excreta and N fertilizer application, the legume-grass mixture emitted 127 compared with 324 kg CO2eq ha-1 yr-1 for N-fertilized bahiagrass. Thus, our results suggest that inclusion of legumes in grass-based systems results in lesser GHG emissions compared with typical N fertilization regimes in the southeastern U.S., indicating legumes are a more sustainable alternative than N fertilizer for production intensification, at least in terms of GHG emissions.
We acquired greater knowledge and awareness of the challenges associated with mitigating greenhouse gas emissions from livestock production and documented that perennial legumes have a significant role to play in this mitigation.
These results indicate that emissions of methane and nitrous oxide from animals grazing rhizoma peanut-bahiagrass pastures are lower relative to those on fertilized bahiagrass pastures, particularly because of emissions related to fertilization use. In addition, emissions of animal excreta are lower than what has been used in estimating emissions from livestock at regional and national reports. Additional work needs to be done to identify emissions of greenhouse gases from excreta of animals grazing cool-season forages in the winter to have annual emissions estimates. Data on enteric fermentation emissions from animals grazing comparable treatments would also significantly improve our ability to identify sustainable management strategies for grazing systems.